Currently available dental adhesive systems for bonding to a dental substrate for various restorative procedures can be classified into two major classes.
The first class is referred to as a total-etch adhesive system. It requires use of an acid etchant, usually a phosphoric acid, to remove the smear layer covering the tooth structure and to demineralize the tooth structure (tooth structure can be either dentin or enamel) so that the primer and adhesive can effectively penetrate into the tooth structure, forming a hybrid layer for enhanced adhesion. A total-etch adhesive system requires a separate acid etchant to etch the tooth structure, and must be rinsed before primer/adhesive application. Because the acid etchant is quite acidic, contact with soft tissue may cause patient irritation or discomfort. The acid etchant removes the smear layer and exposes the dentinal tubules so, if not completely sealed with a primer/adhesive, the open tubules cause sensitivity or discomfort under external stimuli (cold, pressure, etc.) as a result of hydrodynamic fluid movement inside dentinal tubules.
The second class of dental adhesive systems is referred to as a self-etch adhesive system. It eliminates the drawbacks associated with the total-etch adhesive system. That is, a self-etch adhesive system does not require a separate acid etchant, and the self-etch primer/adhesive is not rinsed off so there is less chance of a strongly acidic compound in contact with soft tissues. In the self-etch adhesive system, the primer/adhesive penetrates to and seals wherever it etches, so dentinal tubules are not opened and exposed. With the self-etch adhesive system, there is less technique sensitivity because there is no need to maintain a certain wetness of the dentin surface before applying the adhesive.
The self-etch adhesive system has two subclasses. The first sub-class consists of a self-etch primer and an adhesive, applied sequentially to a tooth structure. The second sub-class combines an etchant, a primer, and an adhesive to form a single-component self-etch priming adhesive system.
While current self-etch adhesive systems bond well to a dentin structure, their bond strength to an enamel structure, especially un-cut enamel, is often inadequate and significantly lower than bond strengths obtained with a total-etch adhesive system counterpart. There is thus a need for a self-etch system that provides improved adhesion to an enamel structure.
Current light-curable dental adhesives, both total-etch and self-etch types, do not work well with self-cured resin cements for indirect applications, especially when cementing a metal-based inlay, onlay, crown, or post. They also do not work well with self-cured composite resins (filling material or core buildup material). One reason is that some systems use an adhesive that does not contain a solvent and thus has a high viscosity, which results in a rather thick adhesive film. A thick adhesive film may interfere with seating an indirect restoration (inlay, onlay, or crown), whereas a thick adhesive film is fine with a direct composite restoration where a composite resin is placed over the adhesive film to fill the cavity and then light-cured. Another reason current light-curable dental adhesives do not work well with self-cured composite resins is that some adhesives contain a high concentration of acidic monomers and therefore are quite acidic, resulting in poor compatibility with a self-curing composite resin (filling material or core buildup material) or a resin cement for indirect restorations, especially when cementing metal-based indirect restoration in which light-curing is limited. The redox initiator system used in most self-cure or dual-cure resin cements or composite resins contains a benzoyl peroxide (BPO) catalyst and a tertiary amine activator. Upon light-curing, the adhesive has a superficial oxygen-inhibited layer containing un-cured acidic monomers. If the adhesive is too acidic, the acid within the oxygen inhibited layer can neutralize the tertiary amine of the resin cement or composite resin coming in contact with the adhesive, thus retarding or preventing the self-curing reaction of the resin cement or composite resin, and resulting in weak adhesion between the self-cured resin cement or composite resin and the acidic adhesive. Another reason current light-curable dental adhesives do not work well clinically is that most self-etching adhesives yield inadequate bond strength to an un-cut enamel structure due to poor etching efficacy of the self-etching primer or adhesive.
Some adhesive systems introduce an additional self-cure activator to improve compatibility between the acidic adhesive and the self-cured resin cement/composite resin. While the efficacy of the self-cure activator is debatable, use of an additional self-cure activator introduces complexity and inconsistency into restorative procedures, e.g., the activator must be mixed with the adhesive prior to application and the adhesive/activator ratio will vary from one application to the next.
Improved dental adhesives are thus desirable.